The objective of this proposal is to determine trace element content in tissues of the rat from birth to death, as well as the subcellular distribution of these elements in brain, liver, kidney, heart and muscle. The elements to be measured are zinc, copper, magnesium, calcium, selenium and manganese. Various enzymes, containing or dependent on several of those elements, will be assayed in brain, liver, kidney, heart and muscle. Changes with aging in trace element content of tissues, subcellular distribution of these elements and enzymatic activities will be studied as well as the relationship of trace element content to the activity of the appropriate enzyme. Additionally, longevity of another group of rats will be increased by underfeeding. These same parameters will be evaluated and compared with the results obtained in the ad libitum fed group.